Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of an image.
Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer,” may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74–75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279–291, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94–103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
Many of the chemicals used to make supports or supported layers in thermally developable materials have electrically insulating properties, and electrostatic charges frequently build up on the materials during manufacture, packaging, and use. The accumulated charges can cause various problems. For example, in photothermographic materials containing photosensitive silver halides, accumulated electrostatic charge can generate light to which the silver halides are sensitive. This may result in imaging defects that are a particular problem where the images are used for medical diagnosis.
Build-up of electrostatic charge can also cause sheets of thermally processable materials to stick together causing misfeeds and jamming within processing equipment. Additionally, accumulated electrostatic charge can attract dust or other particulate matter to the materials, thereby requiring more cleaning to insure rapid transport through the processing equipment and quality imaging.
Build-up of electrostatic charge also makes handling of developed sheets of imaged material more difficult. For example, radiologists desire a static free sheet for viewing on light boxes. This problem can be particularly severe when reviewing an imaged film that has been stored for a long period of time because many antistatic materials loose their effectiveness over time.
In general, electrostatic charge is related to surface resistivity (measured in ohm/sq) and charge level. While electrostatic charge control agents (or antistatic agents) can be included in any layer of an imaging material, the accumulation of electrostatic charge can be prevented by reducing the surface resistivity or by lowering the charge level. These results can usually be achieved by including charge control agents in surface layers such as protective overcoats. In thermally processable materials, charge control agents may be used in backing layers that are on the opposite side of the support as the imaging layers. Another approach taken to reduce surface resistivity is to include a “buried” conductive layer incorporating conductive particles.
A wide variety of charge control agents, both inorganic and organic, have been devised and used for electrostatic charge control and numerous publications describe such agents. Metal oxides are described in conductive layers in U.S. Pat. No. 5,340,676 (Anderson et al.), U.S. Pat. No. 6,464,413 (Oyamada), U.S. Pat. No. 5,368,995 (Christian et al.), and U.S. Pat. No. 5,457,013 (Christian et al.).
U.S. Pat. No. 5,731,119 (Eichorst et al.) describes the use of acicular metal oxides in aqueous-coated conductive layers for use in antistatic compositions. An aqueous-coated sample containing granular zinc antimonate served as a comparison.
U.S. Pat. No. 6,355,405 (Ludemann et al.) describes thermally developable materials that include very thin adhesion-promoting layers on either side of the support. These adhesion-promoting layers include specific mixtures of polymers and other compounds to promote adhesion, and are also known as “carrier” layers.
U.S. Pat. No. 6,689,546 (LaBelle et al.) describes thermally developable materials that contain a backside conductive layer comprising non-acicular metal antimonate nanoparticles in the amount of from about 40 to about 55% (based on total dry weight). These nanoparticles are approximately 20 nm in size. It is believed that upon coating and drying, a physical network of nanoparticles is formed to provide a conductive pathway to remove electrostatic charge.
Conductive layers with a high metal antimonate to binder ratio useful for thermally developable materials are described in copending and commonly assigned U.S. Ser. No. 10/930,428 (filed Aug. 31, 2004 by Ludemann, LaBelle, Koestner, Hefley, Bhave, Geisler, and Philip). Conductivity is provided by non-acicular metal antimonate nanoparticles that are present in an amount greater than 55 and up to 85 dry weight % at a coverage of from about 0.06 to about 0.5 g/m2, and the ratio of total binder polymers in the backside conductive layer to the non-acicular metal antimonate nanoparticles is less than 0.75:1 (based on total dry weight).
Buried backside conductive layers comprising non-acicular metal antimonate nanoparticles in one or more binder polymers, and a non-imaging backside overcoat layer are described in copending and commonly assigned U.S. Ser. No. 10/930,438 (filed Aug. 31, 2004 by Ludemann, LaBelle, Philip, Koestener, and Bhave).
U.S. Pat. No. 6,641,989 (Sasaki et al.) describes photothermographic materials wherein at least one side of the support is provided with a sublayer containing a metal oxide in an amount of 5 to 50% by volume and the surface of the sublayer exhibits a maximum height (Ry) [surface roughness] of not more than 0.1 μm.
The clustering of nanoparticles upon coating and drying to form non-isotropic structures such as chains or trees is described in, Y. Yamaguchi, H. Sasakura, T. Ookubo, and M. Fujita, The Structure Formation of Nanoparticles During Coating and Drying, 12th International Coating Science and Technology Symposium, Rochester, N.Y., Sep. 19–22, 2004, pp. 186–189.
Despite these advances, little attention has been paid to the effect of large-scale processing conditions on the “conductive efficiency” of the formulations described above. For example, when production quantities of backside conductive materials are prepared, the speed required for efficient mixing often results in shear conditions that are different from those involved for the preparation of laboratory quantities. As a result, the conductive backside materials so produced can have properties different from those of laboratory-prepared samples. Often these different properties result in materials having poorer conductive efficiency.
There is, therefore, a continuing need in the industry to find more efficient and less costly ways to reduce electrostatic charge, particularly in “buried” layers on the backside of thermally developable imaging materials. There is also a need for materials with improved conductive efficiency, so that the same performance can be achieved using less conductive material with lower coating weights, and thinner coatings of the conductive layer. There is a further need to prepare these materials by simultaneously coating multiple layers.